Following a series of mutation experiments, using both X-rays and EMS, an extensive cytogenetic analysis of both mutant and nonmutant treated chromosomes is being completed. The contribution of chromosomal rearrangement to lethality, sterility, and visible mutation is being assessed. The distribution of randomly recovered lethal mutations along the X chromosome is being determined by tests with several different duplications, extending over more than a third of the X chromosome, that can be used to "cover" induced lethal mutants. Particular regions that appear to be free of induced lethal effects, or to mutate at an especially low rate, are being identified in the hope of finding evidence for the existence of persisting duplications in the normal genome. Further, the difference between mutant and nonmutant rearrangement breakpoints is being investigated. Also, the detailed relationships of specific genes to particular salivary chromosome bands is being studied, as is the mutability of genes associated with large bands in comparison with that of genes associated with thin, delicate bands. In all experiments, the mutational consequences of treating male germ cells at different stages of maturity is compared (for both chromosomal and nonchromosomal mutants) by testing, separately, each successive brood of progeny produced after treatment. The overall objective is to gain a better understanding of the genetic organization of eukaryotic chromosomes, and to compare the distribution and frequency of EMS-induced mutations with that of X-ray-induced mutations.